Download • The ridges and trenches on the ocean bottom cause corresponding

Mapping the sea • The ridges and trenches on the ocean bottom cause corresponding humps and valleys
on the surface because of variations in the pull of gravity. In 1978 the radar altimeter of a
satellite called Seasat precisely measured the distance to the ocean surface over most of
the globe. Now, using computer processing, geophysicist William Haxby has transformed
that raw satellite data into color maps of the ocean floor. His maps help confirm both
ancient and ongoing movements of tectonic plates and raise intriguing new questions.
Color relief map of ocean floor was created by computer
processing of billions of satellite measurements. Primary
features include: Mendocino Fracture Zone (1); Hawaiian
Emperor Seamount chain (2); Eltanin Fracture Zone (3); Mid­
floor from space By MARCIA BARTUSIAK
"I
t's as if he pulled a giant plug out of the sea bottom
and all the water in the oceans of the world drained
away."
That's the general reaction these days at Columbia
University's Lamont-Doherty Geological Observatory when
inquisitive visitors ask Lamont scientists what they think
of their colleague William Haxby's latest endeavor.
The response doesn't end there. Interest in Haxby's work
Atlantic Ridge (4); Rio Grande Rise (5); Falkland-Agulhas
Fracture Zone (6); Reykjavik Ridge (7); Walvis Ridge (8);
has quickly spread beyond the gates of the sprawling ob­
servatory complex that lies along the scenic banks of the
Hudson River in Palisades, N.Y. At a recent meeting of
the American Geophysical Union, Haxby's presentation
created a sensation among the conferees.
What has Haxby done to engender all this excitement?
After 18 months of painstaking computer processing, the
33-year-old Lamont geophysicist has converted reams of
satellite data into an exquisitely detailed panorama of the
ocean floor-an elaborate color map that identifies underContinued
Agulhas Basin Ridge (9); newly discovered India-Antarctica
Fracture Zone (10). Continents are the solid dark areas.
Satellite gravity map (top) filled in gaps that remained from
bathymetric surveys from ships. Bathymetric map (middle)
shows finer detail but not fractures beneath sediment;
gravity map does. Close-up of the Eltanin Fracture Zone
82
I POPULAR SCIENCE
from Haxby's gravity map (top of frame, above left) has
red spots added to pinpoint past earthquakes. Bottom frame
is a bathymetric map of the area. Above: Gravity map reo
veals sea floor east of Hawaii (upper left of frame).
"Gravity piles water over seamounts" water structures as small as 20 miles across. He did this
by calculating how the sea-floor topography gravitation­
ally tugged at the water above, causing variations in the
height of the sea surface that had been measured by
satellite. The geophysics community views the feat with
astonishment.
Although still in its infancy, such satellite-aided grav­
ity mapping has become a powerful and multifaceted tool
for oceanographers and marine geologists. Researchers
Timothy Dixon and Michael Parke at NASA's Jet Propul­
sion Laboratory in Pasadena, Calif., for example, have al­
ready generated similar maps from the same data. Their
independent venture is enhancing the larger-scale features
of the ocean floor.
Sea-floor mapping, of course, is not a new endeavor. For
decades, mariners have charted the submarine landscape
by taking echo-soundings of the depths below as their ves­
sels crisscrossed the ocean surface. Even gravity maps of
limited areas have been made from shipboard with preci­
sion instruments called gravimeters. But until the Lamont
and Jet Propulsion Lab maps came spewing out of com­
puters, the worldwide picture was far from complete. Large
regions of the South Pacific and South Atlantic sea floors
remained very sketchy. Many geologists have remarked,
only half-jokingly, that the arid landscape of Mars was
better known than that 70 percent of Earth's surface hid­
den beneath a watery veil.
"Let me put this into context," explains geophysicist John
LaBrecque, Haxby's colleague at Lamont. "It would take
several billion dollars and centuries of time for a research
vessel to acquire the amount of knowledge contained in
Bill's maps. They're virtually a television scan of the ocean
bottom ."
had been raked by a giant pitchfork. These are fracture
wnes, the scars left behind as the edges of the plates scrape
past and away from one another. Costly ship surveys have
tracked them before, but never so many or so clearly.
"They're like railroad tracks," explains LaBrecque, "that
now enable us to trace the past movements of the conti­
nents more accurately than ever before." Adds Haxby, "You
can actually follow the motions back to Pangea," that
single land mass that existed some 200 million years ago
when the Americas, Africa, Antarctica, and Australia were
fastened together.
"Take the fracture zone extending from the tip of Af­
rica to the Falkland Islands off the coast of Argentina,"
suggests Haxby. "If you rotate the African continent along
this fracture, you can see how Africa once tucked in quite
nicely next to South America."
Haxby credits the Seasat satellite for this astounding
clarity. With the launching of the instrument-packed probe
in the summer of 1978, oceanographers were obtaining
their first worldwide monitor of ocean activity. Unfortu­
nately, an electrical short circuit terminated the Seasat
mission after only three months, but not before an on­
board radar altimeter had measured the distance between
the spacecraft and the ocean surface to an accuracy of
two to four inches over most of the globe.
"The altimeter sent out a thousand pulses each second
and recorded the time it took for those radar pulses to
bounce off the sea surface," says Haxby. "It did this contin­
uously until the satellite failed." The resulting data-some
eight billion readings in all-enabled Haxby virtually to
X-ray the ocean bottom. How? The answer begins with
gravity's varying strength, determined by local variations
in Earth's mass, both on the continents and beneath the
seas.
Crustal collisions
Scientists have long known that the sea would not be
Indeed, Haxby's maps of the world's sea floors reveal a perfectly level even if the wind stopped blowing and the
terrain as diverse as any found on the seven continents waves ceased rolling. Instead, the surface would subtly
and reinforce geology's most unifying concept: plate mimic the contours of the landscape below. This is be­
tectonics, an idea forged more than a decade ago ["Plate cause gravity tends to pile water above massive underwa­
Tectonics-A Startling New View of Our Turbulent Earth," ter structures such as mountains and ridges because its
PS, June '721. This revolutionary theory says that Earth's strength is greater near such large concentrations of mass.
crust is divided into some 20 plates. Propelled by the tur­ Conversely, the seas subside over objects of lesser mass,
bulent motion of Earth's scorching interior, some of these such as trenches and basins. "A 1,600-foot seamount, for
giant slabs collide, thrusting mountains upward and dig­ example, elevates the sea surface by about eight inches,"
ging trenches deeper. The miles-deep trenches that rim notes Haxby. A miles-deep trench, on the other hand,
the western Pacific, for instance, mark where the huge causes the ocean to drop by dozens of feet. But don't look
Pacific plate is diving into the mantle with a vengeance. for these roller-coaster effects on your next voyage; such
Other plates move apart, giving molten magma deep within bulges and dips are spread out over many miles. Fortu­
Earth the chance to rise and form new sea floors. Such a nately for Haxby, the Seasat satellite, from its lofty perch
process is going on right now at the prominent Mid­ 500 miles out in space, was able to discern the broad varia­
Atlantic Ridge, where the North American plate is inexo­ tions quite easily.
rably separating from the Eurasian and African plates by
The Seasat data arrived at Lamont more than two years
ago on 24 computer tapes. "My original intention," re­
about one inch each year.
Haxby's successful reproduction of these familiar sea­ calls Haxby, "was to map only a small area, the poorly
floor landmarks gave him confidence in his technique, surveyed South Atlantic between Africa and South Amer­
which he has dubbed "geotectonic imagery." But the real ica." His method: converting the varied sea-surface heights
excitement grew when his relentless effort to sharpen the recorded on the tapes into their corresponding gravity
resolution of his maps began to reveal long-hidden features, "signatures," the variations in the force of gravity due to
especially in poorly charted sections of the ocean floor. the presence of underwater structures. Gravity "highs"
New ridges, rifts, and underwater volcanoes (or seamounts) would pinpoint the mountains and ridges; gravity "lows,"
seemed to pop up everywhere. Needless to say, such dis­ the trenches and fractures.
coveries are of more than academic interest. "The Navy is
In fact, Haxby's processing revealed much more. His
certainly interested in discovering new seamounts so their · gravity maps also identified many features hidden beneath
submarines won't run into them," quips Haxby.
hundreds of feet of sediment, because the sediment has
More intriguing were the long, dark streaks cutting relatively little mass. This enables researchers to detect
Continued
across many of the sea floors, as if the muddy bottoms
FEBRUARY 1984
I 83
"Tectonic plates can bend like rubber" long-buried fracture zones not seen on conventional sea­
floor maps.
Focusing the image
Haxby displayed his first results in a standard but very
crude form, as black-and-white contour maps only com­
prehensible to experts. Even then, the Lamont scientists
sensed the power of the technique. LaBrecque remembers
one night of computer processing in particular: "Bill was
sitting at his terminal, cranking in finer and finer con­
touring. We asked him to crank it up some more. He
replied, 'Well, there might be a lot of noise in there.' But
to everyone's surprise, more resolution emerged. It was
like focusing a microscope. Things came out of the ocean
floor that we never had hoped to see."
The most exciting event for Haxby and LaBrecque dur­
ing that all-nighter was finding a thin, sinuous line snak­
ing southward in the Agulhas Basin, several hundred miles
southwest of the tip of Africa. For the first time, geolo­
gists were viewing the ancient mid-ocean ridge that formed
when South America, Africa, and Antarctica started to
spread apart more than 100 million years ago, a time when
dinosaurs still roamed the earth. The buildup of sediment
had kept this fossilized sea-floor-spreading center well
concealed.
"The boundary between the plates later jumped some­
where farther west, so this ridge was left behind to sub­
side," explains Haxby. But it continues to rest on the sea
bottom as a vital piece of evidence that will help research­
ers trace the evolution of the oceans, a process crucial to
the development of climatic patterns.
Such revelations in the South Atlantic prompted Haxby
to expand the scope of his project. "We were perfectly happy
with the contours," recalls LaBrecque, "but Bill went
around mumbling, 'Maybe I can make a picture out of
this.' "
Indeed he could. Soon he was producing a single map of
all the world's ocean floors, using a sophisticated color­
graphics system. He chose to form his photo-like image
out of eight million picture elements (pixels). Each of these
pixels was assigned one of 15 colors to depict the gravity
strength at that particular point, from dark blue (gravity
KM
;[
------------­
M
]
Raw data from Seasat (bottom solid line) show how the sea
surface dips by one meter (3.3 ft.) for a few hundred kilome­
ters over the Mendocino Fracture Zone off the coast of north­
ern California. Haxby fed this data into a computer and
profiled the topography of the sea floor (top solid line).
1b confirm its accuracy, he compared results with measure­
ments of the sea-floor topography done by shipboard depth
soundings (dotted line). The two are a close match.
84
I POPULAR SCIENCE
low) to yellow or pink (gravity high). Haxby also used 256
degrees of shading to highlight the relief. "It acts as if
there is a light source to the north," he points out.
Such touches are more than sheer theatrics. "Color and
shading add a whole new dimension. They bring out fea­
tures not seen in standard contour maps," says Haxby.
Already, certain textures in the color image may provide
a glimpse into Earth's interior. Haxby and Lamont geo­
physicist Jeff Weissel suspect that the faint lines running
like a comb through the central Pacific sea floor may be
the result of convection currents churning 30 to 90 miles
beneath the ocean bottom in the upper mantle. In their
model, each circulating loop consists of hot material ris­
ing and cooler material sinking back into the depths. "A
single ship measuring this area would barely notice this
feature," contends Haxby. Analyzing this relatively small
thermal process might lead to better understanding of the
monstrous convective cells suspected to be driving the
plates themselves.
Accordion squeeze and a rubber plate
A recent newsletter put out by Lamont described geo­
tectonic imagery as providing marine geologists with "a
quantum leap in mapping power." It's an apt description.
The inventory of structures that have been either discov­
ered or better resolved with the observatory's maps in­
creases daily:
• A newly revealed fracture zone in the southern In­
dian Ocean promises to shed light on India's break from
Antarctica 200 million years ago. This 1,000-mile-long
gash, located southwest of the Kerguelen Islands, was
gouged out of the sea floor as the Indian continent inched
northward. India's cataclysmic meeting with Asia 160 mil­
lion years later forged the Himalaya mountains.
• A strange series of east-west wrinkles in the ocean
crust just south of India and Sri Lanka verifies that the
Indian plate continues to push northward, resulting in
earthquakes and surface deformations. "The entire crust
is being squeezed in like an accordion," says Haxby. "This
is something that is not well understood, since the plate
was expected to be more plastic."
• The intense gravity low surrounding the distinctive
Hawaiian Emperor Seamount chain in the mid-Pacific
clearly indicates how a tectonic plate can bend like a rub­
ber mat under the massive weight of a seamount. The
crust beneath Hawaii (the southeast tip of the chain)
deflects by as much as six miles.
• The sharp bend in the lengthy Mendocino Fracture
Zone that juts out of northern California confirms that
the Pacific plate abruptly changed the direction of its mo­
tion millions of years ago. This may be one of the resound­
ing effects of India slamming into Asia.
• The darkened area off the coast of Somalia in north­
west Africa outlines what may be the oldest oceanic crust
on Earth. This gravity low has formed as the aged plate
sinks into the mantle.
• The gravity map confirms that the Reykjavik Ridge,
southwest of Iceland, is indeed V-shaped, a matter of some
controversy. This wedge seems to be forming as hot magma
pulses up from Earth's interior.
Missing high
Even a missing feature is very telling. The prominent
East Pacific Rise-the long ridge system that separates
the Pacific, Antarctic, and Nazca plates off the west coast
of South America-barely shows up on the gravity map.
TRANSMITTED PULSE
(3GHz)
WAVE
TROUGH
~
Radar altimeter aboard Seasat sent 1,000 three-GHz pulses
each second. Pulses traveled 500 miles to the sea surface
and bounced back to the spacecraft. The exact round-trip
travel time of each pulse gave the sea-surface height to an
accuracy of two to four in. Return signal reflects character­
istics of sea surface. Each pulse sampled an area five miles
long and one mile wide. The altimeter averaged each 100­
pulse group and transmitted result to tracking stations.
"This is because the ridge is spreading relatively quickly,
on the order of six inches a year-three times faster than
the Mid-Atlantic Ridge," explains Haxby. "With this rapid
spreading, the materials formed get distributed in such a
way that the gravity signature gets smoothed out."
Scientists around the country are starting to use these
gravity maps of the world's ocean floors to assess and refine
certain geological theories. LaBrecque claims that the im­
ages have already made him change his mind about the
evolution of two prominent features on the South Atlan­
tic ocean floor.
Geologists have long believed that the Rio Grande Rise,
a massive underwater plateau off the southeast coast of
Brazil, and the Walvis Ridge near southwest Africa were
built up long ago when both the South American and Af­
rican plates moved over a "hot spot" beneath the ocean
crust-a region near the Mid-Atlantic Ridge where mol­
ten magma could easily gush out of the mantle below.
But the gravity map, contends LaBrecque, teils another
story. It suggests that the Rio Grande Rise and Walvis
Ridge were once joined, part of a fledgling mid-ocean ridge
system that burgeoned as South America and Africa
started to spread apart.
"The boundary between the plates jumped to the east,
and these structures started to move apart some 100 mil­
lion years ago," ventures LaBrecque. "That intense grav­
ity low on Bill's sea-floor map, south of the Rio Grande
Rise, is the scar that was left behind." As the scenario
goes, the Walvis Ridge continued to rift, "a rift that broke
the African plate apart like a tear in a piece of paper,"
says LaBrecque. On the gravity map this rifting, seen as
a long rivulet of yellow, makes Angola look as if it had
sprung a leak.
No one expects these new satellite charts of the ocean
floor to replace on-site measurements. Most likely the sat-
ellite gravity maps will be used for reconnaissance, cheaply
and efficiently identifying regions where detailed ship sur­
veys of the ocean bottom are most needed. "The accuracy
of a satellite radar altimeter will never match the ac­
curacy of a shipboard gravimeter [an instrument that
directly measures the force of gravity]," says Haxby. "Gravi­
meters can measure a millionth of the force that we nor­
mally feel pulling on us."
Topex: accurate to an inch
Haxby expects his maps to be updated and improved as
new radar altimeters are launched into space by both
NASA and the European Space Agency. He's the first to
admit that his initial effort was far from perfect: "There
were fairly large gaps in Seasat's orbital path; some of
the gaps were as much as 120 miles wide." To compensate,
Haxby had to identify dominant topographic trends and
interpolate the data along those trends.
The Navy's Geosat satellite, scheduled for launch in the
fall, will provide the first opportunity to fill in the miss­
ing pieces. Detailed altimeter information from the first
phase of Geosat's mission will be classified (gravity varia­
tions can affect the flight of missiles), ''but the second phase
shows promise of improving the Seasat data," notes Haxby.
And at the end of the decade, NASA plans to orbit a satel­
lite called Topex (Topographic Experiment), which will be
designed to sense sea-surface variations of less than an
inch. With such capabilities at hand, researchers should
be able to produce gravity maps distinguishing sea-floor
features as small as eight miles across.
For now, however, Haxby is anxious to put his computer
"hacking" to rest so that he can concentrate on more in­
teresting matters: joining his colleagues in interpreting
the many intriguing features being revealed for the first
time in these remarkable maps.
II!J
FEBRUARY 1984
I 85